Currently, in the United States alone, approximately 500 million tons of asphalt paving material mixtures are produced in any given year. Generally, only about 100 tons of asphalt mixtures are tested for quality purposes. As a result, laboratory testing of these mixtures greatly contributes to the design of high performance asphalt pavements.
There are many known different paving material testing machines and methods. Examples of such machines and/or methods are disclosed in the following patents, the teachings of which are hereby incorporated in their entirety by reference:
U.S. PAT. NO.INVENTOR2,972,249McRae et al.3,478,572McRae et al.4,502,338Smith et al.5,036,709McRae5,275,056Hamilton et al.5,456,118Hines et al.5,606,133Hines et al.5,817,946Brovold5,911,164McRae5,916,504Edwards, Jr. et al.5,939,642King et al.
A common objective of known material testing machines is to subject test specimens to conditions which simulate actual use. For paving material test specimens, this requires simulation of the kneading forces applied to the paving materials by the tires of the vehicles passing thereover. It is generally understood in the art that simply applying a compressive force to a test specimen does not adequately simulate the kneading action of vehicular traffic. As a result, compaction machines that gyrate a test specimen during compression have been developed to better simulate actual conditions of use.
An example of such a gyratory compactor is the Gyratory Testing Machine (GTM) developed by the United States Corps of Engineers. Other examples of gyratory compactors are shown and described in some of the U.S. patents identified above. It is generally understood that gyratory compactors are effective tools in evaluating paving materials such as hot mix asphalt (HMA). Gyratory compactors generally have the flexibility of being adjusted to simulate various environmental conditions as well as the tire pressures of any traffic type including cars, trucks and aircraft. An important aspect of gyratory compactors is their ability to monitor the change in mixture response with densification under simulated field conditions. It has been shown that gyratory compactors are capable of achieving the ultimate density of paving materials that is actually obtained in the field. It has also been shown that gyratory compactors can be used for mixture design or quality control of paving materials.
Various governmental bodies have established standards for preparing and testing material specimens so that the properties of the test specimens approximate those of the actual material during construction, under use and over time. For example, the American Association of State Highway and Transportation Officials (AASHTO) has developed a standard (TP4-93) for preparing and determining the density of HMA test specimens by means of gyratory compactors having certain specifications. These gyratory compactors are classified as Superpave™ Gyratory Compactors (SGC). This particular AASHTO standard is used to prepare test specimens which simulate the density, aggregate orientation and structural characteristics obtained in the actual vehicular supporting surface when proper construction procedure is used in the placement of the paving material. Moreover, the AASHTO has developed other standards such as standard (MP2-95) which specifies the minimum quality requirements for asphalt binder, aggregate, and HMA for Superpave™ volumetric mix designs.
Current testing procedures use gyratory compactors to test mixture performance based on the number of applied gyrations and measured volumetric properties of the compacted test specimens. The volumetric design procedure measures the percentage of air voids in a test specimen as a function of the amount of compaction applied. It has recently been observed that the use of gyratory compactors for the evaluation of test specimens based on current specifications, particularly volumetric specifications, does not accurately predict expected field performance for paving materials. A vexatious problem, largely unattended in the art, concerns the lack of an apparatus and method to reliably and economically measure mechanical properties of test specimens subjected to a gyratory compaction process to more accurately predict the expected life cycle of any particular pavement mixture. A significant criticism of the current testing procedures utilizing gyratory compactors, particularly the Superpave™ volumetric design procedure, is the lack of a direct measure of mechanical properties of test specimens and the reliance on the control of densification characteristics of test specimens to predict field performance of paving materials. What is needed is an apparatus and method which is capable of measuring mechanical properties of paving material test specimens. What is further needed is an apparatus and method which predicts paving material performance based on the shear resistance of test specimens subjected to a gyratory compaction process.
Some prior art gyratory compactors have attempted to respond to the problem of lacking a direct measure of mechanical properties of paving material test specimens. It is known that certain gyratory compactors measure the force required for maintaining the angle of gyration. Generally, such known gyratory compactors measure the moment applied to a test specimen mold to maintain the gyration angle. Others have hypothesized that the densification curve used in the current volumetric design procedure can be used to estimate the resistance of test specimens to densification using the approximate energy indices as an alternative to a direct measure of shear resistance.
Even so, one problem with the gyratory compactors which measure the force required to maintain the angle of gyration is that the measured force will include the compounding effects of the mechanical components of the gyratory machine. Such effects include, for instance, the mechanical losses of the mold tilting mechanisms of the gyratory compactors. Another problem with these types of gyratory compactors is that they only measure a uni-directional force applied to the test specimens. Thus, these types of gyratory compactors do not provide an accurate analysis of the mechanical performance of test specimens. Yet another problem with these types of gyratory compactors is that they are machine specific and depend on the particular mold and compactor design. Therefore, what might work for one gyratory compactor may not work for another.
Materials such as paving material test specimens subjected to a gyratory compaction process, can absorb mechanical energy in at least two different ways. The first concerns volume change (densification) and the second concerns shape change or resistance to shape change (distortion). A problem with using the densification curve under the volumetric design procedure to estimate the resistance to densification as an alternative to a direct measure of shear resistance is that it is not completely known if densification specifically correlates with distortion. Thus, the determined results may not accurately predict the expected performance of the paving materials.
Notwithstanding the known deficiencies associated with gyratory compaction equipment and processes, the art has not adequately responded to date with the introduction of a gyratory compaction device and process which is capable of directly measuring the resistance of pavement material test specimens to shearing. In addition, despite the recognition of the lack of a direct measure of mechanical properties by current gyratory compactor designs, the art has produced very little in the way of practical techniques for evaluating paving material test specimen performance in terms of internal shear resistance in order to more accurately predict the likelihood of actual paving materials to maintain their serviceability as well as integrity under vehicular loading in the field.